The general availability of public access to the Internet has fueled the proliferation of online information services. Virtually every conceivable type of information can now be found "online," including text, graphics, audio, and even full-motion video. The ability to receive, or transmit, the different types of data, however, is often limited by the inability of the network, or a user's modem, to receive or transmit the data at a sufficiently fast enough rate.
For most users, access to the Internet is accomplished by using a modem connected to the public switched telephone network ("PSTN"). Because the original PSTN was an analog system, conventional modems have used analog methods to communicate with remote systems coupled to the PSTN. The original analog PSTN, however, is increasingly being replaced by a digital network. In most regions, the only remaining portions of the network that use analog transmission are the subscriber, or "local," loops that connect each user to a central office ("CO"); each CO using a digital network to communicate with other COs and, more and more, to communicate with central site (e.g., Internet service provider) modems connected to the PSTN via a digital connection. Conventional analog modems, however, view the entire PSTN as an analog system, even though the signals are digitized for transmission throughout most of the network.
When a user calls a central site digitally-connected modem, the analog signals transmitted by the client (i.e., user's) modem are digitized by an analog-to-digital converter in an analog interface to a digital portion of the PSTN. In the United States and Japan, the ADC is called a "mu-law" (.mu.-law) ADC, named for the technique used to space the analog-to-digital sample points (also called "quantization levels"); in Europe, a different quantization technique, called "A-Law," is typically used. Mu-law is a standard analog signal compression algorithm used in digital communications systems to optimize (i.e., modify) the dynamic range of an analog signal prior to digitizing.
Mu-Law compression is used to optimize the PSTN for traditional voice communications. The wide dynamic range of speech does not lend itself well to efficient linear digital encoding. Mu-law encoding, however, effectively reduces the dynamic range of the signal, thereby increasing the coding efficiency and resulting in a signal-to-distortion ratio that is greater than that obtained by linear encoding. By artificially limiting the sound spectrum to the bandwidth of normal human speech, the network bandwidth required for each call is reduced, thereby increasing the number of potential simultaneous calls. Although this approach works well for voice communications, it imposes significant limitations on data communications.
When a client modem transmits data, it uses a digital-to-analog converter ("DAC") to convert the digital data, or "symbols" representative of the data, into an analog signal. The analog signal is then received and converted back to digital form by a mu-law ADC in the PSTN. If each of the discrete analog signal levels used by the DAC in the client modem is not within the appropriate quantization interval used by the mu-law ADC in the PSTN, however, the transmitted data may not be accurately converted back to digital form by the mu-law ADC. If the mu-law ADC incorrectly converts a transmitted signal, the central site modem will not receive the same data that was transmitted, resulting in communications errors. In some cases, error-checking protocols can detect a communication error and request the client modem to resend the corrupted data. The need to periodically retransmit data, however, can greatly reduce the average data transmission rate, thereby limiting a user's ability to efficiently transmit data.